1. Field of the Invention
The present invention relates to an etching technique for the fabrication of thin (Al, In, Ga)N layers.
2. Description of the Related Art
Gallium nitride (GaN), and its related ternary and quaternary alloys incorporating aluminum (Al) and indium (In), have attracted widespread interest in the fabrication of high power electronics and visible and ultraviolet optoelectronic devices. Unfortunately, two broad challenges exist in the fabrication of useful devices based on group III nitrides.
First, none of the (Al, In, Ga)N group materials can be readily grown in bulk form. As a result, nitride materials are generally grown by complex techniques, including metalorganic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), and molecular beam epitaxy (MBE). Because of the difficulty in fabricating bulk nitride crystals, the vast majority of group III nitride films and devices are grown heteroepitaxially, i.e., on foreign substrates such as sapphire (Al2O3) and silicon carbide (SiC).
Heteroepitaxial growth generally introduces defects into the nitride films, ranging from threading dislocations to cracks and voids, all of which are deleterious to device performance. Numerous techniques have been developed to reduce the densities of various defects that form during heteroepitaxial growth of nitrides. However, most of these techniques involve growth of films that are thick compared to the actual device structures. These processes can introduce additional impurities, point defects, inhomogeneities, and strain in the composite substrate/nitride film structure that can complicate subsequent device fabrication.
A second significant challenge facing group III nitride device fabrication stems from the chemical, mechanical, and refractory robustness of group III nitrides once they are successfully grown. It is frequently necessary to selectively remove nitride material in order to form mesas, cavities, and membranes, or to completely remove foreign substrates from the nitride films. Unlike silicon and conventional III:V semiconductors such as indium phosphide (InP) and gallium arsenide (GaAs), of which chemical etching is readily performed, etching of group III nitrides has proven extremely difficult.
Several dry etching techniques involving ion bombardment of the nitride surface, including reactive ion etching and inductively coupled plasma etching, have been developed for top-down etching of nitride films and structures. These processes are complex and expensive and great care must be taken to ensure that remaining material is not damaged. Furthermore, these techniques are only suitable for vertical etching, and no equivalent dry lateral etching technique exists. As an added complexity, the ability to etch group III nitrides diminishes as the material quality is improved, e.g. through the dislocation reduction techniques mentioned above.
One lateral etch technique has been developed for group III nitrides. In this process, termed photoelectrochemical (PEC) etching, a combination of above bandgap irradiation and formation of an electrochemical cell using dilute hydrogen chloride (HCl) or potassium hydroxide (KOH) electrolytes is used to selectively etch nitride film on the basis of the films' electronic properties. The incident radiation breaks chemical bonds forming photoexcited holes that allow for oxidation and removal of the irradiated material in the electrolyte solution. This process has proven thus far to be the most effective technique for achieving lateral etching of (Al, In, Ga)N structures.
However, PEC etching suffers from several drawbacks. First, it is bandgap selective, meaning that any material having a smaller band gap than the energy of the incident radiation can potentially etch. For example, a PEC etching process designed to remove an InGaN post from a visible optoelectronic device could potentially etch the InGaN quantum wells in the structure as well as the post. Second, the PEC etching process can be cumbersome to perform, requiring masking, electrode formation, fabrication of suitable optical filters, and use of potentially damaging ultraviolet irradiation. Third, the achievable etch rates are comparatively slow, reaching a maximum of a few micrometers per minute.
There exist numerous applications in which it is desirable to remove a thin (Al, In, Ga)N structure from its substrate. For example, growth of AlGaN-containing optoelectronic device structures on SiC substrates is often performed. Unfortunately, the bandgap of SiC is in the near ultraviolet range of the electromagnetic spectrum, and therefore the substrate will absorb light emitted from most ultraviolet emitters, decreasing device external efficiency.
A similar problem occurs when deep ultraviolet light emitters are grown on free standing GaN substrates. Removal of the absorbing substrate and mounting of the device membrane on a transparent carrier wafer could allow for increased device efficiency.
In a second example, improved light emitting diode external efficiency could be realized if microcavity device geometries could be fabricated. The formation of microcavities requires the formation of very thin (˜40 nm-800 nm) III:N membranes on foreign substrates, which is very challenging using presently available substrate removal and lateral etching techniques.
As mentioned above, in order to grow a high quality optical or electronic device in the (Al, In, Ga)N material system the electrically or optically active layers must be grown on top of thick GaN layers (˜4 μm). For microcavity LED fabrication, the thick heteroepitaxially grown GaN template or buffer first must be removed from the growth substrate (usually sapphire) and then thinned using ion bombardment dry etch techniques. Thickness control of the etch needs to be +/−25 nm so as to obtain a high quality microcavity resonator. But, as mentioned previously, there is very little selectivity with dry etching of GaN and precise thickness control is very difficult to achieve.
Another drawback of dry etching is the potential to damage the optically active layers and reduce the device efficiency. Attempts to perform such substrate removals with laser-assisted liftoff have had very limited success and extremely low yield as the device structures are frequently damaged in the liftoff and etch processes. When the devices are not damaged by the liftoff or etching, the thickness has been difficult to control much of the time, generating low efficiency devices unsuitable for today's technological demands. Therefore, a clear need exists for a physically selective vertical and lateral etch technique that may be used for the reproducible formation of thin (Al, In, Ga)N membranes without damaging the nitride structures therein.
A growing body of literature exists on etching of chemically and mechanically robust materials using ion implantation. Ion slicing is a type of epitaxial liftoff process that involves selective etching of a buried sacrificial layer that is formed by ion implantation. A hard material is subjected to a moderate dose of ions, generating a subsurface damage layer. This damage layer becomes more susceptible to chemical attack than the higher quality bulk material.
The first variation of this ion-assisted etching process was applied to diamond at Oak Ridge National Lab in 1995. First, a single crystal diamond was implanted with carbon or oxygen, after which the diamond was heated to graphitize the damage layer. The graphite layer was then removed by etching in hot chromic acid, leaving the surrounding diamond unaffected. A subsequent refinement of this technique involved heating the diamond/graphite sample in the presence of oxygen at a temperature high enough preferentially oxidize that graphite without affecting the diamond.
Ion slicing has attracted particular attention in metal oxide electro-optical ceramics research. There exists a strong demand for nonlinear optical waveguides coupled with conventional microelectronic devices. Unfortunately, it is difficult to grow single crystal waveguide films on semiconductors, but these waveguide materials, including yttrium iron garnet, lithium niobate, and potassium tantalite, are available in single crystal bulk form. These bulk optical materials were implanted with helium (He) ions, then wet etched to detach single crystal films that were bonded to semiconductor substrates either before or after the etching of the sacrificial layer.
Ion implantation of III:nitrides has been studied for a number of years, predominantly for the purpose of influencing the electronic properties of nitride-based device structures. However, two groups have reported on the use of ion implantation to selectively etch gallium nitride.
A group from the Naval Research Lab recently disclosed a process for selectively etching GaN vertically utilizing ion implantation to achieve etch selectivity. They demonstrated a post-growth ion implantation process that can be used to encourage selective etching of GaN by potassium hydroxide-based photoresist developer. They found that, when a selectively implanted GaN sample is exposed to photoresist developer, only the exposed ion implanted regions etch in the solution. Their United States Utility Patent Application Publication No. US 2002/0096496, filed Nov. 29, 2000, published Jul. 25, 2002, by Molnar et al., and entitled “Patterning of GaN crystal films with ion beams and subsequent wet etching,” discusses several applications of their invention, but their claims specifically require that the GaN be ion implanted in an “imagewise” fashion, rather than over entire surfaces. The claims in this patent application also imply that said implantation would be performed after all growth steps had been performed, which would result in damage to device layers that were not masked from implantation. It is also implicit in this patent application that said etching occurs from the upper growth surface, thus making the technique unsuitable for the fabrication of microcavity devices. Therefore, there exists a clear need for a reproducible backside etching technique that minimizes damage to device layers for the fabrication of microcavity devices.
The present invention provides a selective etching technique that can be used for removal of excess (Al, In, Ga)N from the backside of a template without harming sensitive device layers. As noted above, other known ion bombardment etching techniques, such as reactive ion etching and inductively coupled plasma etching, generate knock-on damage in sensitive device layers, and photoelectrochemical etching tends to attack active quantum well layers, as well as the excess template material. All three of these techniques tend to etch different (Al, In, Ga)N facets at different rates, resulting in large scale roughening of the etched surface. Chemical etching of non-implanted (Al, In, Ga)N proceeds far too slowly to be of practical use due to the chemical inertness of nitride compound semiconductors. Achieving planar removal of material on the nanometer scale with mechanical polishing is virtually impossible. The present invention is free of all of these deficiencies and has widespread application. Perhaps the most promising application of the present invention is in the fabrication of optoelectronic devices such as microcavity light emitting structures, as the invention does not expose the device cavity itself to ion bombardment, mechanical stresses, or irradiation, while still providing for the well-defined removal of underlying material.